Color image reproduction systems known in the art, such as that shown in FIG. 1, permit images to be captured by certain image-receptive media and/or devices such as input device 11, possibly digitized and stored, or output on to one or more image-receptive media and/or devices such as output device 13. Color-image signals from input device 11 may be transformed by transform 14 from the input device's image metric (i.e. a color image signal metric defined in terms of the colorimetric properties of the associated image input device) to a color image metric defined in terms of the colorimetric properties of a particular output device such as output device 13. Typically transform 14 involves three-dimensional signal operations in matrix, three-dimensional look-up-table, or other forms. Output device 13 may form a reproduced image from the transformed color-image signals.
Those skilled in the art will recognize that it is often desirable for the color image reproduction system to create a reproduced neutral scale which differs from the neutral scale of the original image. FIG. 2 for example shows the nonlinear relationship between the original scene densities and the densities reproduced by a typical photographic reflection print system. It is well known in the photographic art that such nonlinearities are necessary to compensate for physical effects such as viewing environment flare light, dynamic range limitations of the output device, and limitations of the output medium, as well as to compensate for psychophysical effects such as lateral inhibition, local brightness adaptation, etc.
When the color image reproduction system's color-image signals correspond to red, green, and blue image-bearing signals, the desired reproduced versus original neutral scale relationship may be achieved by passing the individual red, green, and blue image signals through nonlinear transformations such as one-dimensional look-up-tables. Referring to FIG. 3a, these nonlinear transformations 14a may typically operate on color-image signals which are encoded in terms of the input device image metric. Those skilled in the art will recognize that an input device image metric may be defined in terms of the chromaticity coordinates for a set of primary colors such as the red, green, and blue primary set of FIG. 4. Those skilled in the art will also recognize that the effective responsivities for the input device should correspond to the color-matching functions associated with its image metric primaries. FIG. 5a, for example, shows the color-matching functions associated with the red, green, and blue primaries of FIG. 4. In practice, effective spectral responsivities, such as those of FIG. 5b, are achieved by appropriate combinations of the physical spectral sensitivities of the sensing elements of the input device and the signal processing of the device. The color-image signals produced by the input device for object colors having chromaticity coordinates within region 41 of FIG. 4 would correspond to positive signal values for each color signal. The color-image signals produced by the input device for object colors having chromaticity coordinates outside region 41 of FIG. 4 would include at least one negative color signal value.
Referring now to FIG. 3b, alternately, the nonlinear transformations 14a may typically operate on color-image signals which have been operated on by transform 14 and are encoded in terms of the output device image metric (FIG. 3b). Those skilled in the art will recognize that an output device image metric may be defined in terms of the chromaticity coordinates for the image-forming primaries for the device which, for example, may correspond to the red, green, and blue primary set of FIG. 6. Those skilled in the art will recognize that the output device's gamut of reproducible chromaticities would be limited to the region 61 of FIG. 6 for an output device having these image-forming primaries.
Pictorial images typically contain neutral objects and colored objects illuminated at a range of illumination levels, such as a series ranging from shadow light to highlight. FIG. 7a shows a CIE 1976 metric chroma C* versus CIE 1976 metric lightness L* diagram depicting such a series for a given neutral object 70 and a series for a given colored object 71. A color image reproduction system, such as the systems of FIGS. 3a and 3b, utilizing the nonlinear neutral relationship of FIG. 2 will render the original neutral series 70 of FIG. 7a as shown in 72 of FIG. 7b. Because the nonlinear neutral relationship is achieved in either of these color image reproduction systems by applying nonlinear transformations 14a to the individual red, green, and blue image-bearing signals, the reproduction of a shadow-to-highlight series for colored objects is also altered. This altered color reproduction can result in both desirable and undesirable effects. For example, the reproduction of the original colored object series 71 depicted in FIG. 7a may exhibit C* versus L* characteristics represented by the series 73, 74, or 75 of FIG. 7b.
In the series depicted by 73, as the series progresses toward the highlights (higher L* values), the reproduced chroma of the color decreases while its reproduced lightness increases. In the series depicted by 74, as the series progresses toward the highlights, the reproduced chroma and lightness of the color both increase to certain limits and remain at those limits. In the series depicted by 75, as the series progresses toward the highlights, the reproduced chroma of the color increases while its reproduced lightness also increases. While the effect depicted in series 73 produces visually-pleasing reproduced images, the effects depicted in series 74 and 75 produce undesirable effects in reproduced images.
Our research has shown that when the nonlinear neutral scale transformation is applied to color-image signals corresponding to the colorimetric properties of the selected input or output device, as is typically done in imaging systems known in the art, the reproduced shadow-to-highlight series for some colored objects will correspond to the characteristic depicted in series 73 while for other colored objects, the reproduced shadow-to-highlight series will correspond to the undesirable characteristics depicted in series 74 or 75.
Our research has further shown that:
1) when the nonlinear transformations are applied to color-image signals which are encoded in terms of the input or output device image metric, and the color-image signals describe an object color within the chromaticity gamut defined by the colorimetric properties associated with the input or output device, the modified color-image signals representing a shadow-to-highlight series for that object color will correspond to the desirable characteristic depicted in series 73; PA1 2) when the nonlinear transformations are applied to color-image signals which are in terms of the input or output device image metric, and the color-image signals describe an object color approximately coincident with the chromaticity gamut defined by the colorimetric properties associated with the input or output device, the modified color-image signals representing a shadow-to-highlight series for that object color will correspond to the undesirable characteristic depicted in series 74; and PA1 3) when the nonlinear transformations are applied to color-image signals which are in terms of the input or output device image metric, and the color-image signals describe an object color beyond the chromaticity gamut defined by the colorimetric properties associated with the input or output device, the modified color-image signals representing a shadow-to-highlight series for that object color will correspond to the undesirable characteristic depicted in series 75. PA1 a. transforming the color-image signals corresponding to the first set of color primaries to form intermediary color-image signals corresponding to a third set of color primaries; PA1 b. applying a transform to said intermediary color-image signals to form modified intermediary color-image signals; and PA1 c. transforming said modified intermediary color-image signals to form color-image signals corresponding to said second set of color primaries.
Because practical imaging output devices and media have limited chromaticity gamuts and because nonlinear neutral scale transformations are typically applied to positive-valued color-image signals defined in terms of the colorimetric properties associated with the particular input or output device, the undesirable shadow-to-highlight series effects of 74 and 75 will occur for some object colors. These undesirable effects are particularly noticeable when the chromaticity gamut associated with the colorimetric properties of the actual output device/medium is larger than the chromaticity gamut associated with the positive-valued color-image signals of the color image metric in which the nonlinear transformations are applied.